Current-limiting fuse having parallel current-limiting elements and a series connected current calibrated element with auxiliary arc gaps to blow the current-limiting elements one by one



Feb. 14, 1967 s. l. LINDELL 3,304,387

CUHRENT-LIMITING FUSE HAVING PARALLEL CURRENT-LIMITING ELEMENTS AND A SERIES CONNECTED CURRENT CALIBRATED ELEMENT WITH AUXILIARY ARC GAPS TO BLOW THE Filed Jan. 24, 1966 CURRENT-LIMITING ELEMENTS ONE BY ONE 2 Sheets-Sheet l 56 54 I 56' 54 w 55 l 53 l 52 5/ l5 2/ l7 6 52 l6 l4 49 2/ ,5; 72

47 f1. 2 7/ X1: ,551 65 68 I v l CL) 3,304,387 LIMITING CURRENT CALIBRATED IARY ARC GA Feb. 14, 1967 s. l. LINDELL CURRENT-LIMITING FUSE HAVING PARALLEL CURRENT ELEMENTS AND A SERIES CONNECTED ELEMENT WITH AUXIL PS TO BLOW THE CURRENT-LIMITING ELEMENTS ONE BY ONE Filed Jan. 24. 1966 2 Sheets-Sheet 2 E s U F d 5 m E M 3 5 w 5 R OE RL TT EA m M UR OE T T CURRENT- LIMITING 234 ELEMENTS IN PARALLEL MELTING T. C. CURVE OF CURRENT LIMITING SECTION AVAILABLE CURRENT FZMEEDQ MELTING T. C. CURVE OF RATED ELEMENT COORDINATED WITH CURRENT LIMITING SECTION TO RELEASE INDICATOR THROUGHOUT THE CURRENT RANGE TIME (t) ALTERNATING CURRENT (LOG SCALE) United States Patent Ofiice 3,304,387 Patented Feb. 14, 1967 3 304,387 CURRENT-LHVIITING FUSE HAVING PARALLEL CURRENT-LIMITING ELEMENTS AND A SERIES CQNNECTED CURRENT CALIBRATED ELE- MENT WITH AUXILIARY ARC GAPS TO BLOW THE CURRENT-LIMITING ELEMENTS ONE BY ONE Sigurd I. Lindell, Northbrook, Ill., assignor to S & C Electric Company, Chicago, Ill., a corporation of Delaware Filed Jan. 24, 1966, Ser. No. 522,676 15 Claims. (Cl. 200120) This invention relates, generally, to fuses and it has particular relation to current-limiting fuses.

Among the objects of this invention are: To provide for blowing successively several parallel connected current-limiting fusible elements of a current-limiting fuse, particularly at low fault currents; to avoid melting of the current-limiting elements at their intermediate sections ahead of other sections at low over currents in long time intervals thereby to establish ampere rating and initiation of arcing; to avoid the generation of excessive heat at intermediate sections of the current-limiting fusible elements; to connect the parallel connected current-limiting fusible elements to a common terminal in series circuit relation with ampere rated or current calibrated fusible element means which always blows first to establish a gap on flow of low fault current; to provide a spark gap for each current-limiting fusible element in parallel with the gap that is formed on blowing of the series connected ampere rated or current calibrated fusible element means, each spark gap being in series with the respective current-limiting fusible element and an extension thereof connected to the common terminal; to employ the are formed at random at one of the spark gaps for dividing the flow of fault current between the currentlimiting fusible element individual to the spark gap and the associated extension thereof to cause them to blow, the extension first to separate or disconnect the associated current-limiting fusible element from the common terminal and then at the total fault current, to cause another spark gap to break down and repeat the blowing of the remaining extensions and their current-limiting fusible elements one by one, particularly on low fault currents; to tension the fusible element means to lengthen the gap formed on blowing thereof; to provide a movable indicator that is moved from non-indicating position to indicating position by a spring that tensions the current calibrated fusible element means and, when it blows, lengthens the resulting gap; and to employ at least three current-limiting fusible elements each having substantially uniformly spaced reduced cross sections along their lengths with their total time-current characteristic spaced substantially above the time-current characteristic of the current calibrated series fusible element means at the long time end to reduce heat generation and to restrict the operating temperature of the current-limiting fusible elements under normal sustained load conditions and with the long time end of the time-current characteristic of an individual current-limiting element lying substantially below the time-current characteristic of the current calibrated fusible element means so that the former will blow at the minimum current caused to flow through its ex tension on low fault current operation.

In the drawings: FIG. 1 is a vertical sectional view substantially at full scale of a current-limiting fuse embodying this invention. FIG. 2 is a view, similar to the upper end portion of FIG. 1, and showing the fuse in the blown condition with the indicator extended. FIGS. 3, 4 and are horizontal sections taken, respectively, generally along lines 3-3, 4-4 and 5-5 of FIG. 1. FIG.

6 is a top plan view of the ceramic core shown in FIG. 7. FIG. 7 is a view, in side elevation, of the ceramic core shown in FIG. 6. FIG. 8 illustrates diagrammatically the circuit connections embodying the current-limiting fuse shown in FIG. 1. FIG. 9 shows graphically the relation between the melting times of the various current carrying elements of the current-limiting fuse for different current flows therethrough. FIG. 10 shows graphically the current flow when the available short circuit magnitude of alternating current is so high that the current-limiting section melts on the rise of current and the current flow is interrupted in a fraction of the first current loop by the current-limiting fuse embodying this invention.

In FIG. 1 the reference character 10 designates, generally, a current-limiting fuse which includes an elongated insulating housing 11 of ceramic material such as porcelain. It may be formed of other insulating material. The elongated insulating housing 11 has a fuse upper end terminal indicated, generally, at 12 and a fuse lower end terminal indicated, generally, at 13.

The upper end terminal 12 includes a metallic ferrule 14 that is secured by cement or other means to the upper end of the housing 11. The ferrule 14 has connector plates 15 welded thereto for receiving screws 16 that extend through a radial flange 17 that forms an integral part of a metallic terminal cap 18. The end terminal 12 also includes a metallic tube 19 that extends downwardly into the tubular insulating housing 11 and is provided with a radially inwardly extending flange 20. It will be understood that the metallic ferrule 14, metallic terminal cap 18 and metallic tube 19 are solidly interconnected and that the metallic terminal cap 18 is arranged to be positioned in a fuse clip. Gaskets, shown collectively at 21, serve to revent the egress of relatively inert granular material 22, such as sand, which fills the tubular insulating housing 11.

The lower end terminal 13 includes a metallic ferrule 25 that is suitably secured by cement to the lower end of the insulating housing 11. It has connector plates 26 welded thereto for receiving screws 27 that extend through a flange 28 which extends outwardly from a metallic terminal cap 29. The metallic terminal cap 29 has an opening 30 through which the granular material 22 can be introduced. A closure cap 31 is provided for the opening 30. The lower end terminal 13 also includes a connector plate 32. Gaskets, shown collectively at 33, are employed to prevent the egress of the granular material 22. It will be understood that the ferrule 25, metallic terminal cap 29 and connector plate 32 are solidly interconnected and that metallic terminal cap 29 is arranged to be received by a fuse clip for connection in a circuit which may be energized at a'voltage range upwardly from 2400 volts and in which the normal load current flow may be of the order of 3 to 200 alternating our-rent amperes.

Extending centrally of the tubular insulating housing 11 is a ceramic core that is indicated, generally, at 36 and is shown in more detail in FIGS. 6 and 7. The ceramic core 36 is provided with a central opening 37 and has four vertical ribs 38 that are provided with grooves 39 for receiving in spiral fashion preferably four or more current-limiting fusible elements or ribbons 40 which may be formed of silver. For the lower ampere ratings, when it is desirable to use the greatest practical number of elements in parallel, they are formed of materials having a higher resistivity and higher mechanical strength than silver. For example nickel and alloys of silver can be used. Perforations 41 are uniformly spaced along each of the current-limiting fusible elements 40 to provide localized restricted cross sections to initiate fusion and arcing on flow of fault current. The lower ends of the current-limiting fusible elements or ribbons 40 are connected by terminal clips 42 secured to the connector plate 32 by screws 43. Thus the lower ends of the current-limiting fusible elements or ribbons 40 are solidly connected to the fuse lower end terminal 13.

The upper ends of the current-limiting fusible elements or ribbons 40 extend through terminals 46 which are generally tubular in configuration and are deformed thereon to make good contact therewith. The terminals 46 extend through the central portion of the ceramic core 36 and are connected to a common intermediate terminal, shown generally at 47, which is insulated from the upper and lower end terminals 12 and 13 except as it is connected thereto in the manner described herein. The intermediate terminal 47 includes a tubular section 48 through which the terminals 46 extend and to which they are solidly connected. The intermediate terminal 47 also includes a lower fuse element terminal 49 to which the lower end of current calibrated fusible element means, indicated generally at 50, is connected. The current calibrated fusible element means 50 includes a coiled silver fusible element 51 and a strain wire 52. They are connected at their upper ends to an upper fuse element terminal 53 which extends through an apertured flange 54 of a metallic indicator tube 55 that is movable endwise and centrally of the upper end of the tubular insulating housing 11. A nut 56, threaded on the upper end of the upper fuse element terminal 53, holds the latter in position on the upper end of the metallic indicator tube 55. It will be noted that the metallic indicator tube 55 is movable upwardly through a flanged opening 57 in the upper end of the metallic terminal 18. The non-indicating position of the indicator tube 55 is shown in FIG. 1 while its indicating position is shown in FIG. 2, a position which it occupies after the current calibrated fusible element means 50 has blown. The circuit is completed from metallic indicator tube 55 through contact fingers 58 which are secured by screws 59 to the flange '20 which forms a part of the metallic tube 19. A stop ring 60, suitably secured to the outer surface of the metallic indicator tube 55 intermediate its ends, serves to limit its upward movement in the indicating position as shown in FIG. 2.

The metallic indicator tube 55 is slidable on a guide tube 62 that is formed of ceramic material or other suitable high temperature insulating material. The tube 55 has a radial flange 63 through which the screws 59 extend into the upper ends of the ribs 38 of the ceramic core 46. In order to tension the fusible element means 50 and to move the indicator tube 55 to the indicating position, a coil compression spring 64 is employed. It is interposed between the apertured flange 54 at the upper end of the indicator tube 55 and the upper end of the guide tube 62. To increase the voltage that appears at the gap that is formed on blowing Of-the fusible element means 50, a fuse tube 65 is employed that is formed of a material, such as fiber, or an inorganic material carrying a material which evolves an arc extinguishing gaseous medium when subjected to the heat of an In order to make certain that the current-limiting fusible elements or ribbons 40 blow on flow of relatively low fault current and substantially simultaneously at the numerous perforations 41, special means are provided for causing them to blow sequentially one by one. For this purpose, as shown in FIGS. :1 and 2, a terminal conducting strip 66 is provided for each of the currentiimrt ng fusible elements or ribbons 40 and is connected to a unction 67 thereof between an extension 68 of each current limiting fusible element or ribbon 40 and the ma or portion 69 thereof. The extension 68 is relatively short as compared to the length of the major portion 69 and its resistance is negligible. The arrangement is illust-rated diagrammatically in FIG. 8 where the reference characm 40 Indicates, g n rally, a currentlirnitmg fusible element or ribbon which includes the extension 68 and the major portion 69. As seen in FIGS. 1 and 2, the extension 68, which is the portion between each terminal conducting strip 66 and the respective terminal 46, is embedded in the granular material 22 as is the respective major portion 69. The lower end of each of the terminal conducting strips 66 is crimped onto the respective junction 67 while the upper end carries an electrode 70 that projects into an opening 71 in the radial flange 63 of ceramic material. Each of the openings 71 registers with a vent opening 72 in the flange 76 which extends from the metallic tube 19 and which constitutes the other electrode of a spark gap 73, FIG. 8. A spark gap 73 is connected in series with the respective major portion 69 of each of the current-limiting fusible elements or ribbons 40 providing a current path from each through its gap to the metallic tube 19 and thereby to the upper end terminal 12 while the associated extension 68 provides a current path from the spark gap 73 to the common intermediate terminal 47 and the other current-limiting elements 40. Screws 74 extending through the terminal conducting strips 66 into the respective ribs 38 of the ceramic core 36 serve to hold them in place. A gasket 75 underlies the radial flange 63 of the guide tube 6-2 to prevent egress of the granular material 22.

It will be understood that the several spark gaps 73 normally are shunted by the current calibrated fusible element means 50 as long as it remains intact. However, when it blows and forms a gap 76, FIG. 2, the several spark gaps 73 then are conected in parallel circuit relation with the gap 76. The voltage across the gap '76 is applied across the spark gaps 73 which are arranged to break down at this voltage one at a time at random. Advantage is taken of this characteristic for sequentially blowing the current-limiting fusible elements or ribbons 40 on low fault currents, the extension 68 first to interrupt the current path to the common intermediate terminal 47 and thereafter the associated major portion 69 at the total fault current.

In FIG. 9 curve 78 represents the time-current relationship for the fusible element means 50 which also may be referred to as a rated element or current calibrated element since it can be calibrated to blow as indicated by curve 78 and by its response at the long time end of this curve to determine the ampere rating of the current-limiting fuse 10 which rating is conventionally approximately one-half of the five-minute or ten-minute melting current. Broken line curve 79 indicates the time-current relation ship for the blowing of all four of the parallel connected current-limiting fusible elements or ribbons 40. It will be observed that this curve is entirely to the right of the curve 78 and thus indicates that the arrangement is such that the current calibrated fusible element means 56 blows first on flow of fault curent followed by blowing of the current-limiting fusible elements or ribbons 46. The succeeding curves, shown by broken lines at 80, 81 and 82, show the time-current relationships for the blowing of three, two and one, respectively, of the current-limiting fusible elements or ribbons 40. To simplify the presentation, the curves 78-82 here shown illustrate typical average current values. it is understood that allowance for plus or minus deviations are to be made with suflicient margin for coordination with other series connected current interrupting means and with the fusible sections Within the current-limiting fuse 10. A greater number of current-limiting fusible elements or ribbons 40 in parallel permits the use of smaller cross section individual current-limiting elements for a given current rating and provides a higher speed or response.

Curve 83 in FIG. 10 indicates the current in a half loop of alternating current that is available to flow in the event of a high curent fault provided some means is not introduced to limit such current flow to a value well below the maximum here indicated. Assuming that the current flow is as indicated at 1 in FIG. 9 at time the current calibrated fusible element means 50 blows. This is indicated at A, in FIG. and is followed at 1 by the blowing of the current-limiting fusible elements or ribbons 40, substantially simultaneously. Because the current-limit.- ing fusible elements or ribbons 4-0 are embedded in the inert filling 22, such as sand, the resistance in the circuit provided thereby rises rapidly so that the current, instead of following the curve 33, is rapidly decreased and at time t FIG. 10, is reduced to zero.

Particular difficulty is encountered when the flow of fault current is substantially lower. For example, it is conventional to rate a fuse to blow :at a minimum value of approximately twice its normal current carrying rating. Such a fault current is indicated at I in FIG. 9. In that case it will be observed that the flow of fault current is not sufficient to blow all four of the current-limiting fusible elements or ribbons 41) of the curent-limiting use 16 constructed in accordance with the present invention but it would be SUlfiClBIlt to blow one of them as indicated at t along curve 82. By providing the spark gaps 73 in the manner described, it is possible successively to blow the current-limiting fusible elements or ribbons 40 so that, under the assumed conditions where the current fiow is as indicated at I they will be blown one by one.

After the current calibrated fusible element means 50 has blown, the voltage across the resulting gap 76 is applied across the spark gaps 73 which are connected in parallel. They are so spaced that the voltage thus available will break them down and one of them does so at random so that an arc is formed therebetween, for example as indicated at 84 in FIG. 8. Since the gap 76 has been established by blowing of the current calibrated fusible element means 50, the entire fault current I flows through the random established are 84 with onefourth of the current flowingthrough the major portion 69 of the respective current limiting fusible element or ribbon 40 while three-fourths of the fault current I flows through the associated extension 68 the resistance of which is relatively negligible. The flow of such a magnitude of fault current through the extension 68 is sufiicient to cause it to blow with the result that the entire flow of fault current I takes place through the major portion 69 of the current-limiting element or ribbon 40 where the are 84 was formed and it blows at t as indicated by curve 82.

Assuming that the spark gaps 73 continue to break down from left to right as viewed in FIG. 8, an arc is formed at the next spark gap 73 with one-third of the fault current flowing through the respective major portion 69 and'two-thirds of the fault current flowing through the respective extension 68 with the result that the extension blows and the associated major portion 69 then blows as the result of having the entire flow of fault current take place therethrough.

For the remaining two unblown current-limiting fusible elements or ribbons 40, the flow of fault current I divides evenly and curve 81 shows that this amount of fault current is sufficient to cause them to blow. Curve 82 shows that current in an amount of 1 is sufiicient to cause any one of the current-limiting fusible elements or ribbons 4t) to blow. In the event that one blows before the other, the last to blow is subjected to the flow of fault current I and it blows.

Since the current calibrated fusible element means 5t? always blow on flow of fault current, as indicated by curve 78, the indicator tube 5%) always is released and shows that the current limiting fuse It has operated and should be replaced.

As pointed out, provision is made for blowing the current-limiting fusible elements or ribbons 40 one by one in the event that the How of fault current is relatively low. Then it is unnecessary, as in certain prior art current-limiting fuses, to provide special means along the current-limiting fusible elements or ribbons 40 at their midpoints to initiate melting there to cause them to blow. The current-limiting fusible elements or ribbons 40 operate at a relatively lower temperature and there is less heat generated at their midsections. Also, it is possible to make the current-limiting fusible elements or ribbms 40 in relatively low ampere ratings and with the perforations 41 therein since it is unnecessary to rely on the flow of fault current for blowing all of them more or less simultaneously as is the case when initially they divide the fault current equally.

Since the arrangement is such that the current calibrated fusible element means 50 always blows first, as indicated by curve 78, it is unnecessary to provide for connecting a release element for the indicator tube 55 across the terminals 12 and 13 to respond after the current-limiting fusible elements or ribbons 40 have blown as is required in certain prior art current-limiting fuses. This is particularly important in that it avoids the conventional approach where a resistance element employed for making such a connection is usually coiled and extended through an opening or bore, such as the opening 37, through the ceramic core 36. In low ampere rated current-limiting fuses, this opening is relatively small and difficulty is encountered in finding the necessary space for the resistance element and the required coil spacing and filling material (sand) to avoid cascading of thecoils with resultant collapse of the current suppressing arc voltage.

Employing the construction disclosed herein, it is possible not only to improve the performance of currentlimiting fuses employing silver for the current-limiting fusible elements or ribbons 40 by reducing the operating temperature and causing melting to occur simultaneously along their intermediate sections on flow of elevated current but also to use pure nickel for the current-limiting fusible elements or ribbons 40 to create lower ampere ratings. Nickel can operate continuously at temperatures .below 500 C. without deterioration. For currentlirniting fuses of low ampere rating, the calibrated fusible element means 50 in series can effectively limit the current-limiting fusible elements or ribbons 40 to temperatures below this level. Elimination of a concentration of perforations 41 at the midsections of the currentlimiting fusible elements or ribbons 40 and elevating the long time end of the time-current curve 79 substantially above the long time series melting current represented by curve 78 reduces the heat generation and lowers the operating temperature of the current-limiting section.

Without the special current calibrated fusible element means 50 of this invention, it is diflicult to obtain proper sequential blowing of an adequate number of parallel current-limiting elements 40 in fuses of low ampere ratings for purely mechanical reasons. In these low ampere ratings in the prior art the ribbons become of such small dimensions that it is difiicult to fabricate them. When it is understood that at least four current-limiting elements or ribbons 40 should be used in parallel in order to achieve any degree of adequate fusion of numerous gaps or at numerous points therealong so as to end up with relatively high voltage withstandability along the path of each conductor after blowing, the advantages of this invention become more readily apparent. In certain prior art fuses, when one current-limiting conductor melts out, it simply shifts the current flow to the remaining ones which melt more or less out of step one by one until one of them carries all of the fault current. It actually melts out at an adequate number of points. Its arc voltage then goes back to fiashover the initially established very short gaps in the other current-limiting conductors. This is sometimes a problem where switching surges are involved and there can be no guarantee that the fuse did not clear the initial fault with some of the remaining gaps inadequate to withstand a subsequent switching surge.

In the current-limiting fuse it is clear that the entire fault current is applied to each one of the current-limiting elements or ribbons 40 sequentially in contradistinction to the prior art where only a part of the fault current may be applied to some of the elements thereby leaving gaps which later on are subject to break down and are unable to withstand the voltage that is applied across the fuse terminals. This is the case particularly in those designs where the melting or blowing of the several current-limiting elements is initiated more or less simultaneously at the initial equal current division.

Some of the advantages of the present invention found in the construction of a low amperage current-limiting fuse also are useful in a higher amperage current-limiting fuse where the limits as to mechanical construction are not present. These advantages include the absence of the hot point at the mid section with a corresponding reduced temperature under normal operating conditions and uniform gapping along each current-limiting element or ribbon 40 that goes with a particular kind of initiating blowing which is not apt to leave short gaps along any one of the current-limiting elements or ribbons 40 to be flashed over by a subsequent switching surge or by application of line voltage.

What is claimed as new is:

1. A current-limiting fuse including:

an elongated insulating housing with end terminals,

an intermediate terminal in said housing insulated from said end terminals,

fusible element means interconnecting one end terminal and said intermediate terminal,

a plurality of current-limiting fusible elements embedded in a granular filling in said housing and connected in parallel circuit relation between said intermediate terminal and the other end terminal,

a minor portion of each current-limiting fusible element adjacent said intermediate terminal constituting an extension of a major portion thereof, and

means providing a spark gap individual to each currentlimiting fusible element, one terminal of each spark gap being connected to the junction between the respective extension and the major portion of its current limiting fusible element, the other terminal of each spark gap including conductor means connected to said one end terminal thereby placing said spark gaps in shunt circuit relation to the gap formed on blowing of said fusible element means on flow of fault current therethrough.

2. The current-limiting fuse according to claim ll wherein:

the intermediate terminal is located at one end of the housing, and

the fusible element means is in air and within the end terminal at said one end of said housing.

3. The current-limiting fuse according to claim 1 wherein the spark gap is formed between a pair of spaced electrodes forming the terminals of said spark gap.

4. The current-limiting fuse according to claim 1 wherein the conductor means is a metallic tube extending from the one end terminal into the elongated housing.

5. The current-limiting fuse according to claim 4 wherein:

the metallic tube at its inner end has a radially inwardly extending annular flange forming a common electrode for the spark gaps, and

the terminal connected to the junction between each extension and its current-limiting fusible element includes a conducting strip secured at one end to said junction and having an electrode at the other end spaced from said annular flange.

6. The current-limiting fuse according to claim 1 wherein a spring tensions the fusible element means and lengthens the gap formed on blowing thereof.

7. The current-limiting fuse according to claim 6 wherein an indicator is movable endwise of the elongated housing, is biased to indicating position by the spring, and is restrained in non-indicating position as long as the fusible element means remains intact.

8. The current-limiting fuse according to claim 1 wherein on blowing of the fusible element means the voltage across the resulting gap is applied to the spark gaps one of which breaks down at random to cause part of the fault current to flow through the associated extension causing it to blow, this sequence of operations being repeated until all of the current-limiting fusible elements have blown.

9. The current-limiting fuse according to claim 1 wherein the extension of each current-limiting fusible element is embedded in the granular filling.

It The current-limiting fuse according to claim 9 wherein the intermediate terminal includes a tubular section through the wall of which terminal fittings radially extend for connection to the extensions of the currentlimiting fusible elements.

11. In a current-limiting fuse current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected in series circuit relation with said current calibrated fusible element means,

the current values of the long time end of the timecurrent characteristic curve for said current calibrated fusible element means lying below the current values of the long time end of the time-current characteristic curve for said pluality of parallel connected current-limiting fusible elements and above the long time end of the time-current characteristic curve for one of said parallel connected current-limiting fusible elements, and

means responsive to blowing of said current calibrated fusible element means including a spark gap individual to and connected in series circuit relation with each current-limiting fusible element and connected in parallel circuit relation with said current calibrated fusible elements for causing said current-limiting fusible elements to blow one by one.

12. The current-limiting fuse according to claim 1 wherein the intermediate terminal is stationarily mounted in the insulating housing.

13. The current-limiting fuse according to claim 1 wherein the current-limiting fusible elements are formed of metal having a higher resistivity and higher strength than silver.

14. The current-limiting fuse according to claim 1 wherein each current-limiting fusible element is provided with substantially uniformly spaced reduced cross sections to provide locations at which it blows on flow of fault current.

15. In a current-limiting fuse current calibrated fusible element means,

a plurality of parallel connected current-limiting fusible elements connected by a common terminal in series circuit relation with said current calibrated fusible element means,

the current values of the long time end of the timecurrent characteristic curve for said current calibrated fusible element means lying in a range below the current values of the long time end of the time-current characteristic curve for said plurality of parallel connected current-limiting fusible elements and above the long time end of the time-current characteristic curve for each individual one of said parallel connected current-limiting fusible elements whereby at said long time ends of said curves the current re- 3,304,387 9 10 quired to blow said current calibrated fusible element References Cited by the Examiner means is less than the current required to blow said UNITED STATES PATENTS current-limiting fusible elements simultaneously and is more than is required to blow said current-limiting 2,200,608 5/1940 Triplet; 200 117 fusible elements individually, and 5 21302320 11/1942 Van Llempt 200 135 means responsive to blowing of said current calibrated 2,400,408 5/1946 Plaeflfinger 200 118 fusible element means in said range for interrupting 2,502,992 4/1950 Rawhns et a1 200 120 X said current-limiting fusible elements adjacent said 2,847,537 8/1958 Kozacka 200135 terminal and to establish in series with each an arc 2,879,354 3/1959 Fahnoe 200120 X gap and for maintaining said current through one of 10 3,218,517 11/1965 Sankey 317 66 said current-limiting fusible elements at random to blow it followed by random blowing of the remaining BERNARD GILHEANY Examine current-limiting fusible elements one by one. H. B. GILSON, Assistant Examiner. 

1. A CURRENT-LIMITING FUSE INCLUDING: AN ELONGATED INSULATING HOUSING WITH END TERMINALS, AN INTERMEDIATE TERMINAL IN SAID HOUSING INSULATED FROM SAID END TERMINALS, FUSIBLE ELEMENT MEANS INTERCONNECTING ONE END TERMINAL AND SAID INTERMEDIATE TERMINAL, A PLURALITY OF CURRENT-LIMITING FUSIBLE ELEMENTS EMBEDDED IN A GRANULAR FILLING IN SAID HOUSING AND CONNECTED IN PARALLEL CIRCUIT RELATION BETWEEN SAID INTERMEDIATE TERMINAL AND THE OTHER END TERMINAL, A MINOR PORTION OF EACH CURRENT-LIMITING FUSIBLE ELEMENT ADJACENT SAID INTERMEDIATE TERMINAL CONSTITUTING AN EXTENSION OF A MAJOR PORTION THEREOF, AND 